The field-emission microscope (FEM) was invented by Muller in 1936. In that instrument, an image was formed by applying a high voltage (about 5 kV) between a flat plate detector (fluorescent screen) and a field-emission tip which served as the cathode. Electrons were emitted from the tip by the field-emission process and followed the simple field lines to the anode. This process produces a high-magnification (1 million times) image of the emission region of the tip on the imaging screen. The image resolution is better than 1 nm.
In 1951, Muller introduced the field-ion microscope (FIM). The tip in this instrument is the anode and an imaging gas (hydrogen, or an inert gas) is introduced into the system at low pressure (10.sup.-3 Pa). The principle is similar to FEM but ionized gas species produce the image at the screen. The FIM has the advantage that the image is much brighter on a fluorescent screen. In the 1960's, this instrument was applied to structural studies of metals.
In 1967, Muller, et al., introduced the atom probe field-ion microscope (AP/FIM). In this instrument, the imaging gas is removed and, with the tip operating as an anode, the field is increased to the point where the atoms on the surface are evaporated by the field. Since the process of field evaporation involves ionization of the atoms, they are accelerated to the imaging screen by the applied field. An aperture with a virtual size of 0.5 to 5 nm is placed in the image screen to allow the ions from a specific location on the tip to pass though into a time-of-flight mass spectrometer.
Since time-of-flight information is used to identify the atom, it is necessary to know when it leaves the tip. This is usually accomplished by keeping the applied field low enough that negligible evaporation occurs and then superimposing a very short (in temporal duration) pulse on the high voltage (about 15% overvoltage). The field during the pulse must be high enough to cause evaporation of a specified small number of atoms (0.01 to 10).
Mass resolution in a time-of-flight atom probe varies with the mass of the atom. A typical atom probe which uses pulsing of the field achieves a mass resolution of 1 part in 200 for molybdenum. At this mass resolution, it is not possible to distinguish all of the medium to heavy isotopes in a given material. One approach which overcomes this difficulty is the use of an energy-compensating filter which improves the mass resolution to about 1 in 2000 for molybdenum. Two common energy compensating systems are the poschenreider lens and the reflectron. At mass resolutions of 1 in 2000, it is possible to resolve virtually any isotope except those that have identical mass to charge ratios like N.sup.2+ and Si.sup.1+.
For conventional atom-probe operation, this filter is highly desirable. It has the further advantage that atom-probe studies can be performed with imaging gases present (because the gas ions can be filtered out of the data). This means that high quality FIM images are available during probing. Though such a system is large and complex, it has been used on a commercially available atom probe made by Vacuum Generators of England.
A microchannel plate (MCP) detector which has been used in AP/FIM multiplies the signal produced by ions or electrons at any point in a planar or curved array. It has an adjustable high gain and low noise. This detector produces robust images in FIM mode and it made possible detection of individual ions. Furthermore, when the ions arrive, a signal is produced which can be used for timing.
In 1973, Panitz, Rev. Sci. Instrum. 44, p. 1034 et seq., described an atom probe which measured the time of flight of individual ions from the tip to any point on a detector (image) plane. Though any location on the image plane could be monitored at any time this system did not have true parallel detection. However, the detection system could be used for both imaging and for measuring time of flight. Imaging atom probes (IAP) evolved from this concept. By selectively turning on, or gating, the microchannel plate detector for short periods of time when ions of a selected species are expected to arrive, it is possible to produce an image due only to that species.
In the late 1970's, Tsong, Surf. Sci., Vol. 70, 1978, p. 211, and Kellogg and Tsong, J. Appl. Phys. 51, 1980, pp. 1184 et seq., described the use and advantages of thermal excitation by a pulsed laser for inducing field evaporation. They showed that the field required to induce a given evaporation rate (0.5 monolayers per second) is inversely related to the temperature of the tip. By holding the field just below the critical evaporation field and then heating with a short (7 ns) laser pulse, they were able to induce controlled evaporation in a pulsed mode. The major advantage of this approach is that the applied field is constant during the pulse. This obviates the need for a Poschenreider lens. If the thermal transient is short, then the time during which the ion may leave the tip is determined with high precision. Kellogg and Tsong achieved a mass resolution of about 1 in 100 at molybdenum with the laser that they employed. A mass resolution of about 1 in 3000 at tungsten (about 1 in 2000 at molybdenum) has been shown to be achievable with this pulsed-laser atom probe (PLAP).
A parallel detection scheme has been applied to the atom probe by a group at Oxford University, as described in Patent Cooperation Treaty Application PCT/GB86/00437, publication No. W087/00682, and A. Cerezo, et al., Rev. Sci. Instrum. 59 (1988) p. 862 et seq. A parallel detector for an atom probe must determine two pieces of information: a) the identity and b) the location on the tip of each atom (ion) that is evaporated from the tip. The Oxford group used a wedge and strip (WSA) detector at the output of the microchannel plates. See E. Martin, et al., Rev. Sci. Instrum. 52 (1981), pp. 1067 et seq. The atom's identity is determined by the time of flight from the tip to the microchannel plate. The location of the atom on the tip is determined from the position where the charge cloud hits the WSA. Using voltage-pulsing, the Oxford group has produced position sensitive atom probe (POSAP) images wherein several phases present on a nanometer scale are apparent and their composition is determined exactly. The addition of a parallel detector to the atom probe thus changes the output of an atom probe from a linear composition profile to a volume composition image.
While the benefits of POSAP for microanalysis on the atomic scale are clear, current instrumentation suffers from several limitations. The most notable drawbacks are the low data collection rate (system throughput) and relatively low mass resolution.
The limits on the ion count rate in the current POSAP stem from the nature of the detector and the frequency of the desorption pulses. Like most position-sensitive detectors, the wedge-and-strip detector is a serial counting device. This means that there is a time constant associated with detecting and processing each ion event. If a second ion arrives at the detector during the time that the first is being processed (of the order of the time constant), then the charge from the two events will be summed and the reported position will be inaccurate. The rate of data acquisition is therefore restricted by the requirement that the probability that two atoms evaporate per pulse becomes negligible only when the probability that one atom evaporates per pulse is on the order of 0.01-0.03. There is also a conversion time for the pulse-processing electronics described below, the time constant for which is on the order of 300 ns for the detector followed by a 60 .mu.s conversion time for the analog-to-digital conversion (ADC). This suggests a maximum pulsing rate of 1 per 60 .mu.s or about 2 .times. 10.sup.4 per second. Since one in 50 pulses produces an ion, and the ADC can be cleared in approximately 2 .mu.s during a null event, the ion counting rate can actually be determined by 50 null events plus one detection per ion, which is 160 .mu.s per ion or 6 .times. 10.sup.3 ions per second.
Because the field is changing during the rise and fall of a high-voltage pulse, the energy of a departing ion will depend on when during the pulse the ion leaves. This spread in energy of the ions translates directly into a spread in the flight times of ions and the corresponding mass resolution is decreased. To prevent excessive spreads in the ion energy, the high voltage pulse applied to the tip must have a sub-nanosecond rise time. The techniques for providing these pulses are limited, and can only provide frequencies in the 100 to 250 Hz range. The pulsed-field position-sensitive atom probe (PFPOSAP) can therefore be pulsed no faster than about 250 Hz. At one atom per 50 pluses, this translates into a data collection rate of about 5 atoms per second. Thus, the speed of the PFPOSAP is limited by the pulsing since the detection process can run about 10.sup.3 times faster. A typical FIM image is about 100 atoms across. To collect an image which is 100 atoms on a side by 100 atoms deep (20 nm).sup.3, it takes about 30 to 50 hours. Clearly then, it is desirable that the pulse rate be more closely matched to the throughput of the detector system to speed up the POSAP.
This throughput limitation can be overcome by using an alternative method to desorb ions from the tip: heating the tip several hundred Kelvin with a short pulse of energy. The advantages of pulsed evaporation by thermal assist are manifold and clear. They have largely bern elucidated by work on the pulsed-laser atom probe (PLAP). However, though pulsed lasers have been used with good success, they have yet to be applied at high repetition rate in short-flight-path instruments like POSAP. See G.T.L. Kellogg, et al., J. Appl. Phys. 51 (1980) pp. 1184 et seq.; T. T. Tsong, et al., Rev. Sci. Instrum. 53 (1982) pp. 1442 et seq.; W. Drachel, et al., Int. J. Mass. Spectrom. Ion Phys. 32 (1980), pp. 333 et seq.; A. Cerezo, et al., J. of Microscopy 141 (1986), pp. 155 et seq.; A. Cerezo, et al., J. Phys. (Orsay) C9 (1984) C9, pp. 315 et seq.
A high pulsing speed is physically possible but it will not be useful unless the detection system can operate at comparable speeds. Typical position-sensitive detector systems are only capable of detecting about 6 .times. 10.sup.3 events per second. Two basic types of position-sensitive detector exist: a) those based on charge separation, and b) those based on a binary signal (hit or not). Charge separation on electrodes gives good position information but is limited in speed by the charge integration electronics. Furthermore, the detectors generally handle only one event at a time. The binary-result detectors can be high speed and can handle simultaneous events, however, the precision of the binary-result detectors can be no better than the wire spacing. There is also the problem of determining the position when several adjacent wires are hit.